THIS INVENTION relates to densifying of a bulk particulate material. In particular, it relates to a method and to apparatus for densifying a bulk particulate material.
According to a first aspect of the invention, there is provided a method of densifying a bulk particulate material, the method including
at least partially confining the bulk particulate material; and
rotating a rotatable member submerged under the bulk particulate material about an axis of rotation to cause movement of the material particles essentially towards or away from the axis of rotation, thereby to provide a densified bulk particulate material.
Thus, at least on initial contact of the material particles with the rotatable member, the particles move generally towards or away from the axis of rotation.
Confining the bulk particulate material may include feeding the bulk particulate material into a vessel. Typically, the vessel has a wall defining a circular cylindrical interior surface. The vessel may have a central, longitudinal axis which is coaxial with the axis of rotation of the rotatable member.
The method may include vibrating the vessel to inhibit agglomeration or build-up or caking of the particulate material against interior surfaces of the vessel.
The method may include discharging the densified bulk particulate material from the vessel. It is to be appreciated that the method can be conducted on a continuous basis or on a batch basis, discharging of the densified bulk particulate material from the vessel and feeding of bulk particulate material into the vessel thus occurring batch-wise, or on a controlled basis. Thus, the bulk particulate material may be fed on a continuous basis into the vessel, and the densified bulk particulate material may be discharged on a continuous basis from the vessel.
The method may include measuring or determining the bulk density of the densified bulk particulate material prior to discharging it from the vessel. Instead, the method may include measuring or determining the bulk density of the densified bulk particulate material after it has been discharged from the vessel.
The method may include controlling the density of the densified bulk particulate material. The controlling of the density of the densified bulk particulate material may be effected by a method selected from the group consisting of manipulating the residence time of the bulk particulate material in the vessel, manipulating the angular speed of rotation of the rotatable member, manipulating the level of the bulk particulate material in the vessel, or two or more of these methods The controlling of the density of the densified bulk particulate material is however not necessarily limited to these methods.
The axis of rotation of the rotatable member may be substantially vertical. Preferably, the rotatable member causes movement of the material particles inwardly towards the vertical axis of rotation. In another embodiment of the invention, the coaxial axis of rotation and longitudinal axis of the vessel are at an angle of about 60xc2x0 to the horizontal.
The rotatable member may be rotated at an angular speed-of between 100 rpm and 3500 rpm. Preferably, the rotatable member is rotated at an angular speed of between 500 rpm and 1000 rpm. Typically, the rotatable member is rotated at an angular speed of between 700 rpm and 800 rpm, e.g. about 732 rpm.
The bulk particulate material may have a mean particle size of less than 1 mm. Typically, the bulk particulate material has a mean particle size of less than 0.5 mm, even less than 1 xcexcm, e.g. about 0.15 xcexcm.
The method may include extracting dust from the vessel.
The bulk particulate material may be particulate silica having a particle size of the less than 0.5 xcexcm, typically less than 0.2 xcexcm. Indeed, it is expected that the invention will find particular, though not necessarily exclusive application in densifying so-called silica fume.
The ratio of the density of the silica prior to densifying thereof, to the density of the densified silica may be at least than 2:3. Preferably, the ratio of the density of the silica prior to densifying thereof, to the density of the densified silica is at least 1:2, depending on the density of the silica prior to densifying. The ratio can be as large as 1:3, or even larger, depending on the density of the silica prior to densifying.
According to a second aspect of the invention, there is provided apparatus for densifying a bulk particulate material, the apparatus including
a vessel for at least partially confining a body of the bulk particulate material;
a rotatable member which is arranged such that in use it is submerged in the body of bulk particulate material and causes movement of the material particles essentially towards or away from an axis of rotation when the rotatable member is rotated about the axis of rotation; and
drive means connected to the rotatable member and capable of rotating the rotatable member when it is submerged in the body of bulk particulate material.
The vessel may have an outlet for densified bulk particulate material at a low elevation, and an inlet for bulk particulate material at a higher elevation than the outlet. The rotatable member may be located between the inlet and the outlet of the vessel. Preferably, the rotatable member is located at the elevation of the outlet of the vessel.
The drive means may be capable of rotating the rotatable member at an angular speed of between 100 rpm and 3500 rpm when the rotatable member is submerged in the body of particulate material. Typically, the drive means is capable of rotating the rotatable member at an angular speed of between 500 rpm and 1000 rpm when the rotatable member is submerged in the body of particulate material, e.g. at about 700 rpm to 800 rpm.
The rotatable member may include a plurality of circumferentially spaced vanes. The vanes may be directed or arranged in use to displace the bulk particulate material generally radially relative to the axis of rotation of the rotatable member. Preferably, the vanes are directed or arranged so that the movement of the material particles is inwardly towards the axis of rotation when the rotatable member is rotated, at least on initial contact of the material particles with the rotatable member.
The rotatable member may include a disk-shaped body from which the vanes project. The vanes may project from a surface of the disk-shaped body which is an upper surface in use. Instead, the vanes may project radially outwardly from a periphery of the disk-shaped body.
The vanes may be planar and may be substantially tangential to a drive shaft connecting the drive means to the rotatable member. A radially inner end portion of each vane may be truncated so that the radially inner end of the vane forms an angle of between 15xc2x0 and 60xc2x0 with the axis of rotation in the plane of the vane. Preferably, the angle is between 20xc2x0 and 50xc2x0, e.g. about 30xc2x0.
The vessel may have a wall defining a circular cylindrical interior surface, and a central, longitudinal axis which may be coaxial with the axis of rotation of the rotatable member. The ratio of the diameter of a circle described by the rotatable member when it rotates, to the diameter of the vessel may be between 0.25:1 and 0.99:1. Preferably, the ratio is between 0.5:1 and 0.99:1. Typically, the ratio of the diameter of the circle described by the rotatable member when it rotates, to the diameter of the vessel is between 0.9:1 and 0.99:1, e.g. about 0.95:1.
The vessel may have a volume of between 0.1 m3 and 200 m3 . Typically, the vessel has a volume of between 0.1 m3 and 0.5 m3.
The axis of rotation of the rotatable member may be substantially vertical.
The apparatus may include conveying means and bagging means, the conveying means being arranged to convey densified bulk particulate material from the vessel to the bagging means for bagging the densified bulk particulate material.
The apparatus may include vibration means for vibrating the vessel to inhibit agglomeration or caking or build-up of the particulate material against interior surfaces of the vessel.
The apparatus may include dust extraction means for extracting dust from the vessel.
The rotatable member and interior surfaces of the vessel may be coated with a material which inhibits caking or agglomeration or build-up of the bulk particulate material against or on them.
The apparatus may include density measurement means and control means for controlling the bulk density of the densified bulk particulate material.